Electric discharge device



June 27, 1961 R. P. WELLINGER EI'AL 2,990,492

ELECTRIC DISCHARGE DEVICE Filed 001;. 24, 1958 FIG.|.

United States Patent 2,990,492 ELECTRIC DISCHARGE DEVICE Roger P. Wellinger, Scotia, and James E. Roderick,

Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Filed Oct. 24, 1958, Ser. No. 769,381 14 Claims. (Cl. 31354) Our invention relates to electric discharge devices and pentains more particularly to new and improved gap devices which are adapted for improved stability and reliability in operation and for uniform long operating life.

Many gap devices are employable for protective purposes such as, for example, to protect capacitors from overvoltage in power transmission systems. In view of the fact that overvoltage of the system can result in substantial and costly damage to the system and components therein, operation of a gap device which will insure rapid shorting of a circuit in response to an overload condition is highly desirable. However, in order to avoid unnecessary interruptions in the operation of a circuit, it is also highly desirable that the device be effective for bolding off or avoiding discharge until an offending load condition prevails. In connection with the stability of operation, it is also desirable to avoid excessive over-shoot of the rising potential or, in other words, undesirably high increases in potential beyond that at which the device is adapted for discharging. Further, it is desirable that the device be adapted for rapid recovery or extinguishment following a discharge so as thus to restore the system to normal rapidly and to stand ready to discharge again in response to a subsequent overload condition. Still further, uniform long operating life is highly desirable from both the standpoints of cost and continuous reliable protection of systems employing such devices. Additionally, it is desirable from the standpoints of ease and cost of manufacture to provide gap devices which by their construction aiford all of the above-mentioned desiderata and yet are simple in construction and can be easily assembled by mass production techniques.

In prior art devices radioactive ionizing agents have been employed to affect operational characteristics. Where such agents are employed it is desirable to provide devices which meet the safety requirements of governmental regulations and yet are reliable and commercially feasible.

Accordingly, a primary object of our invention is to provide new and improved gap devices adapted for improved reliability and stability by being operative to discharge more rapidly in response to overvoltage or overload conditions and to avoid discharge at less than overload conditions in systems protected by such devices.

Another object of our invention is to provide new and improved gap devices including improved means for avoiding spurious discharges between elements of different potentials therein, thus to increase stability of opera tion and to afford rapid recovery following an operational discharge.

Another object of our invention is to provide new and improved gap devices including improved means for maintaining a contained ionizable medium in a predetermined ionized condition, thus to improve reliability and stability in operation.

Another object of our invention is to provide new and improved gap devices containing ionizable mediums and improved means for safely and commercially practically pre-ionizing same by radioactive means.

Another object of our invention is to provide new and improved gap devices including improved arrangements of elements adapted for increasing the effective operating lives of said devices while assuring substantially uniform operation throughout such increased operating lives.

Further objects and advantages of our invention will become apparent as the following description proceeds and the features of novelty which characterize our invention will be pointed out with particularity in the claims annexed to and forming part of this specification.

In carrying out the objects of our invention we provide a gap device comprising an enclosed envelope containing an ionizable medium and radioactive means for maintaining same normally in a predetermined level of ionization. The envelope includes a single-walled hollow insulator having a metal cap with a flared rim bonded to the outer surface at one end of the insulator. Bonded in the cap is an electrode support member extending through the insulator in spaced relation thereto. The outer surface of the insulator is annularly grooved for increasing the electrical distance thereacross and sealed to the other end of the insulator is a metallic can having the rim flared and bonded to the outer surface of the insulator. Provided centrally in the bottom of the can is an active electrode surface which cooperates with a similar surface on the inner end of the support member for defining the smallest electrical gap in the device. The active surface of the can is defined by an indentation in the bot-tom of the can. A mounting plate on the bottom of the can serves both to mount the device and dissipate heat therefrom. Additionally, the electrode support member is massive and of high specific heat capacity for comprising a heat sink. Still further, the electrode on the support member is dimensioned for shielding the insulator from metal sputtering from an arc in the gap defined by the electrode and the indentation in the bottom of the can.

For a better understanding of our invention reference may be had to the accompanying drawing wherein:

FIGURE 1 comprises a sectional elevation view of a gap device constructed in accordance with a form of our invention; and

FIGURE 2 is a fragmentary sectional view of a modifled form of radioactive element employable in our device.

Referring to FIGURE 1, there is shown a gap device generally designated 1 and constructed in accordance with one embodiment of our invention. The device 1 includes an enclosed metal and ceramic envelope 2 containing an ionizable medium adapted for being normally maintained at a predetermined level of ionization below that which prevails during an. operational discharge in the device. The nature of this medium and the means provided for ionizing same will be brought out in detail hereinafter.

The envelope 2 includes a high refractory ceramic section or insulator 3 which can advantageously be of the alumina type. The insulator 3 is elongated and hollow or tubular and consists of a single wall, the particular purpose of which will be brought out in detail hereinafter. The outer surface of the insulator 3 is formed to include a plurality of annular recesses 4 which serve to increase substantially the electrical leakage path between the ends of the insulator.

Fitted on the upper end of the insulator 3 is a cupshaped metal cap 5. The bottom of the cap is spaced longitudinally from the upper end of the insulator and the rim of the cap is flared and sealed by means of a suitable metallic ceramic-to-metal bond 6 to the outer surface of the insulator.

The bottom of the cap 5 is apertured, and brazed therein and to an outwardly projecting lip 7 is a metal support rod 8. This arrangement of hermetically sealing and supporting the rod 8 on one end of the insulator disposes all sharp metal edges outwardly of the envelope, and, thus, minimizes any tendency toward spurious arcing which could have the deleterious effects of causing premature discharge and retarding recovery or extinguishment of an operational discharge. Additionally, the illustrated arrangement places the seal between the cap and insulator in compression for greater sealing reliability.

The rod 8 is elongated and extends both inwardly and outwardly of the envelope. Suitably fitted and secured in the outer end of the rod 8 is an end of a conductor or lead 9. The lead 9 can be flexible and have a lug 10 secured on the outer opposite end thereof. The inner end of the rod 8 extends concentrically through the insulator 3 and is spaced therefrom throughout the length of the rod. Additionally, the inner end of the rod extends beyond the inner end of the insulator and carries thereon an electrode element 11 which can be spherical as shown for providing a rounded active electrode surface 12. It is to be understood, however, that the element 11 need not be spherical but can have any configuration sutficient for providing the desired rounded end surface and the desired shielding and heat capacity to be discussed in greater detail hereinafter.

The active surface 12 is disposed oppositely of a rounded active electrode surface 13 defined by an inward indentation 14 formed centrally in the bottom of a cuplike metal member or can generally designated 15. The can 15 can be fabricated of a generally cylindrical wall member and a bottom plate including the indentation 14. This construction provides for ready dissipation of heat from the electrode formed by the indentation 14 and, by enabling the can to be used as an electrode economically, avoids the necessity of providing a pair of electrode elements such as the one designated 11. Additionally, by using the metal can it is possible to take advantage as ionization producing means any electrons that may be knocked out of the metal of the can body by particles impinging thereupon.

The upper rim 16 of the can 15 is flared and is bonded to the outer surface of the insulator 3 by a suitable ceramic-to-metal bond 17 which is also in compression. This bond is effected intermediate the ends of the insulator such that the inner end 18 of the insulator, which may be referred to as an apron, extends substantially into the can 15. The lower rim of the wall of the insulator extends outwardly and is suitably brazed to the metal plate including the indentation 14 for completing the can. This disposes the sharp edges outwardly of the envelope. The indented plate at the bottom of the can is provided with a metal tubulation 20 through which the device can be first evacuated and then charged with a predetermined gaseous medium. The tubulation is adapted for being sealed by pinching and is provided with a protective cap 21 containing a quantity of soft solder for insuring sealing of the tubulation and absorbing shock. Additionally, the bottom plate includes a lateral extension or arm '22 which can be apertured for facilitating mounting of the device and is also effective for assisting in dissipating heat. The material of the can 15 can, for example, advantageously be copper, copper-clad chromeiron, or mild steel.

The above-mentioned heat is generated by an are which, during normal operation, occurs in the gap defined by the active electrode surfaces 12 and 13. This gap can be in the approximate range of from 0.025" to 1" and is the shortest unobstructed distance between opposed electrodes of opposed polarity in the device for thus increasing the tendency that all arcing in the device will occur across this gap. The purposes for the rounded configuration of the active surfaces 12 and 13 are several fold. First, the rounded surfaces afford the greatest region of field concentration in the device and, thus, a stronger tendency for the operational discharge are to travel between the central extremity of the electrode 12 and the center of the bottom of the can 15 rather than to any inadvertent granule or minute sharp protrusion on the side wall of the can. Thus, the device is more reliable and stable in that the arc will tend always to occur uniformly in the predetermined fixed gap. Secondly, the opposed rounded active surfaces 12 and 13 are effective for equalizing the breakdown voltages in both polarities. Thus, in an alternating current circuit the device can be relied upon for breaking down or dis charging and recovering substantially uniformly at given potential values regardless of direction of current flow. Thirdly, the rounded active surfaces cause the arc to be confined to a relatively small central area in the can and coaxially beneath the electrode element 12. This, together with the relatively large dimensions of the electrode 11, has the desirable effect of shading or shielding the inner end or apron 18 of the insulator from deposition of evaporated metal resulting from the arc. Evaporated metal, if any, is instead deposited harm, lessly on the metal side walls of the can. The shielding is of both the internal surface of the insulator and the external surface of the apron 18. If arcing were to occur elsewhere than in the gap between the active sur faces 12 and 13 such as, for example, between the electrode 11 and the side walls of the can, evaporated metal would tend to deposit on the ceramic with the undesirable result of rendering the internal surfaces thereof conductive and subject to cracking, which could result in failure and substantially shortened life. The spacing between the electrode support rod and inner wall of the ceramic results in an increased insulator length internally of the envelope and which is substantially shaded from sputtering conductive material from the gap. This increases the effectiveness of the insulator in the device.

As also seen in the drawing, the support rod 8 and the electrode element 11 are substantially massive relative to the other metal parts of the device. This is to afford substantial specific heat capacity or ability to take on a substantial quantity of heat. The heat from a normal operational discharge must be dissipated. Thus in the structure illustrated, we employ a massive, or large compared to other metal elements, electrode and support rod formed of a material of high specific heat capacity. Too small an electrode cannot be used since, for example, if the diameter of the electrode were on the order of the size of the are spot practically no thermal conductivity would be available, and additionally, the PR loss in the electrode would result in additional heat and the entire electrode would quickly melt away.

The size of the electrode element 11 relative to the are spot is important, also, since, as pointed out above, the electrode 11 is relied upon to some extent for blocking any evaporated metal from the are spot which might tend to deposit on either the inner or outer Walls of the ceramic insulator. Thus, the electrode element 11 is dimensioned for blocking entrance of evaporated metal into or onto the insulator and for deflecting such metal outwardly for harmlessly depositing on the side walls of the can.

Additionally, and as pointed out above, the device 1 is adapted for containing an ionizable medium. This, for example, can advantageously be a gas such as argon, or helium at a pressure desirably from .01 mm. of Hg to 2500 mm. of Hg. In order to maintain this medium at a predetermined level of ionizationbetween discharges, thus to assure reliable firing of a discharge without excessive over-shoot of the rising potential, we provide radioactive means for preionizing the medium or ionizing it in advance of an operational discharge.

In the drawing is shown an annular strip or band of Nickel-63 designated 23. This radioisotope is effective for providing from .1 to 10 microcuries of radioactivity. Nickel-63 is particularly advantageous to safe use because (1) of its long half-life compared to anticipated tube life which means that the tube need not initially contain an unsafe amount of radioactive material and (2) the relatively low energy Beta emission of Nickel-63 will not penetrate the tube walls. The Nickel- 63 can comprise a strip secured in place in the manner shown and by any suitable means, such as tabs (not shown). This concentric arrangement of the Nickel-63 immediately about the gap desirably concentrates radiation from the Nickel-63 on the ionizable medium in the p- If it is desired to use a strip of Nickel-63, it can be obtained by subjecting .a strip of nickel to neutron bombardment in an atomic pile which will result in conversion of part of the nickel to Nickel-63 and Cobalt-60. A ring such as the ring 23 in the drawing can then be fabricated from the irradiated material or the nickel could be fabricated as a ring before irradiation. For some purposes, however, this process may currently prove costly. Additionally, this process creates a Gamma emitter, Cobalt-6O in the ring material which it is generally desirable to avoid including in commercial devices. Another method which we have found suitable for purposes of commercial devices is the plating of Nickel63 onto a ring from a plating solution containing Nickel-63, and to provide an annular structure such as that fragmentarily illustrated in FIGURE 2. In this structure the Nickel-63 comprises a plated layer 25 on a suitable metallic annular base or ring 26. The concentration of Nickel-63 in the plating should be great enough toprovide the permissible radioactivity in a layer sufficiently thin to assure negligible attenuation of Beta radiation by the plated layer. In other words, the Nickel-63 layer is desirably thick enough to provide all the radiation desired but is not so thick as to hinder substantially Beta radiation from the Nickel-63 below the surface of the layer. The optimum thickness can be determined from the standpoint of negligible attenuation of Beta radiation and the area of the layer can then be adjusted for obtaining the desired quantity of radiation. The material of the ring 26 can be selected from any of those materials which facilitate nickel plating. For example, copper rings have been used and found satisfactory.

If desired, a radioactive ionizable agent can be used in place of the Nickel-63 and to provide the desired preionization. We have successfully employed Krypton- 85 for providing approximately 5 microcuries of radioactivity to effect the desired preionization. A gaseous radioactive material such as Krypton85 is advantageous in a gap device in that it is, unlike a solid radioactive material, not subject to being covered by metal evaporated from the active electrode surfaces. Thus, it adapts the device for longer, more uniformly operative life. Additionally, should leakage occur, the radioactive gas would escape harmlessly. Also, no disposal problem is present since the device can be rendered harmless simply by allowing the radioactive gas to escape to the atmosphere. Still further, we have found that Krypton-85 is effective for inhibiting arc anchoring or, in other words, it minimizes any tendency of the arc to remain stationary in the gap which generally has the undesirable tendency to burn spots in the active electrode surfaces. It is believed that the Krypton-85 somehow is effective to foster movement of the are over the active electrode surfaces.

As pointed out above, the device is constructed to avoid sharp edges or corners inside the envelope and to make all seals externally of the envelope with the use of flared rims on the sealed metal parts. This avoids undesired concentration of electrostatic fields which, with the presence of an ionizing particle such as that afforded by the radioactive means in the envelope, would tend to allow a breakdown or discharge at a lower voltage than that at which the device is adapted for normally discharging. Additionally, such lower voltage breakdown would not be consistent because of the selective presence of ionizing particles. That is, if a sharp edge or corner were present in the envelope, the density of ionizing particles, not being infinite, would not always assure the presence of a particle near an edge at the same instant that desired or operational breakdown voltage is reached and, therefore, breakdown could often initiate at some other distance than the gap distance between surfaces 12 and 13 and at some other breakdown voltage than the normal operating voltage. Additionally, it is generally difficult to obtain uniform ionizing particle density especially in view of the contribution of corona to ionization. Thus, it is desirable to strive for uniform ionization as by avoiding internal sharp edges and internal corona just before normal breakdown, which internal corona would add to the relatively uniform ionization caused by the radioactive means. For best operation the maximum field in the device should occur in the gap defined by the active electrode surfaces 12 and 13 which is obtained in the present device.

It will be understood from the foregoing that in addition to improved operational characteristics, higher reliability, and stability in operation and increased uniform operational life, our device is simple in construction and adapted for easy assembly with uniform results by use of mass production techniques.

While we have shown and described a specific embodiment of our invention we do not desire our invention to be limited to the particular form shown and described, and we intend by the appended claims to cover all modifications within the spirit and scope of our invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

l. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated singlewalled hollow insulator, a coaxial cup-like metal sealing cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, a rounded active electrode surface on the end of said support member opposite said cap, said rounded surface being large relative to the opening in said hollow insulator disposed outwardly of the corresponding end of said insulator, a coaxial metallic cup-like member having the rim thereof bonded to the outer surface of said insulator opposite said cap, an oppositely rounded active electrode surface formed centrally in the bottom of said cup-like member defining a gap with said active surface on the end of said support member, and said gap being shorter than the distance between said active surface and the side walls of said cup-like member.

2. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated singlewalled hollow insulator, a coaxial cup-like metal sea-ling cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap, said support member extending concentrically through said in sulator in spaced relation thereto throughout the full length of said support member and being the only member in said insulator, an active electrode surface on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, a coaxial substantially flat-bottomed metallic cuplike member having the rim thereof bonded to the outer surface of said insulator opposite said cap, an inward indentation located centrally in the bottom of said cuplike member defining a gap with said active surface on the end of said support member, and said gap being shorter than the distance between said active surface and the side walls of said cup-like member, an inert ionizable medium in said envelope at a pressure of from .01 mm. of Hg to 2500 mm. of Hg, and radioactive means contained in said envelope maintaining said medium ionized at a predetermined level lower than the normal level of ionization at discharge of said gap.

3. A gap device comprising a sealed envelope including a metal wall section and containing an ionizable medium, a pair of longitudinally spaced opposed active electrode surfaces in said envelope defining a gap, a quantity of radioactive material in annular form supported on the inner surface of said metal wall section and extending in a plane transverse the axis and at the axial level of said gap for concentrating radiation on the portion of said ionizable medium in said gap.

4. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated singlewalled hollow insulator, a coaxial cup-like metal sealing cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode surface on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, a coaxial metallic cup-like member having the rim thereof bonded to the outer surface of said insulator opposite said cap, in inwardly extending active electrode surface formed centrally in the bottom of said cup-like member defining an axial gap with said active surface on the end of said support member, and said gap being shorter than the distance between said active surface and the side Walls of said cup-like member, an inert ionizable medium in said envelope, and a quantity of Nickel-63 disposed on the inner side wall of said envelope effective for maintaining said medium predetermined pre-ionized.

5. A gap device according to claim 4 wherein said Nickel-63 is in the form of an annular strip and is concentrically supported on the side walls of said coaxial metallic cup-like member in a plane transverse the axis and at the axial level of said gap to concentrate radiation on the ionizable medium in said gap.

6. A gap device according to claim 4 wherein said Nickel-63 is in a layer supported on the internal side walls of said metallic cup-like member and has a thickness and area to provide a predetermined quantity of radioactivity with negligible attenuation of Beta radiation from Nickel-63 below the surface of said layer.

7. A gap device comprising a sealed envelope containing an ionizable medium, said envelope including a metal cup-like member having an inwardly indented bottom, an electrode extending in said cup-like member and insulatedly mounted on the rim thereof, said electrode having a rounded active surface cooperating with said indentation in said member for defining a gap, and a quantity of radio-active gas for pre-ionizing said medium.

8. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated single walled hollow insulator, a coaxial cup-like metal sealing cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode element on the end of said support member opposite said cap, said element being of greater diameter than the opening through said insulator and disposed outwardly of the corresponding end of said insulator, a coaxial metallic cup-like member of greater diameter than said insulator and having the rim thereof bonded to the outer surface of said insulator opposite said cap, an active electrode inwardly extending surface formed centrally in the bottom of said cup-like member defining a gap with said active surface on the end of said support member, and said gap comprising the shortest distance and the greatest region of field concentration between opposed metal surfaces supported from opposite ends of said insulator, and said envelope containing an atmosphere of an ionizable medium, part of which is radioactive.

9. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated single-walled hollow insulator, a coaxial cup-like metal sealing cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, said support member being the sole element in said insulator and substantially filling said insulator except for the spacing therebetween, an active electrode surface on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said in sulator, a coaxial metallic cup-like member having the rim thereof bonded to the outer surface of said insulator opposite said cap, indentation located centrally in the bottom of said cup-like member defining a gap with said active surface on the end of said support member, and said gap comprising the shortest distance and the greatest region of field concentration between opposed metal surfaces supported from opposite ends of said insulator, and said envelope containing a quantity of radioactive Krypton effective for maintaining the medium in said envelope predeterminedly pre-ionized.

10. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated singlewalled hollow insulator, a coaxial cup-like metal sealing cap having the rim thereof bonded to the external lateral surface of one end of said insulator, an elongated metal electrode support member bonded to said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode surface on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, a coaxial metallic cup-like member having the rim thereof bonded to the outer surface of said insulator opposite said cap, an inward indentation located centrally in the bottom of said cup-like member defining a gap with said active surface on the end of said support member, said gap being shorter than the distance between said active surface and the side walls of said cup-like member, and a mounting bracket extending laterally from the bottom of said cuplike member.

11. A gap device comprising a sealed envelope, having as wall-defining elements thereof an elongated singlewalled hollow ceramic insulator, a metal sealing cap bonded across one end of said insulator, an elongated metal support member mounted on said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, said active electrode being large relative to the opening in said corresponding end of said insulator, a coaxial metal cup-like member having the rim thereof bonded to the outer surface of said insulator intermediate the ends thereof and with the inner end of said insulator extending a substantial distance into said cup-like member in spaced relation to the inner side walls thereof, an active electrode surface formed centrally in the bottom of said cup-like member defining a gap with said active surface on the end of said support member, said gap defining the shortest distance and the greatest region of field concentration between opposed metal surfaces supported from opposite ends of said insulator, and the major portions of the surfaces of said insulator disposed inwardly of said envelope being shielded from the deposition of evaporated metal from an arc in said gap as a result of the concentric disposition of said gap, the inward extension of said inner end of said insulator and the relative dimensions of said active electrode and said corresponding end of said insulator.

taining an ionizable medium and having as Wall-defining elements thereof an elongated single-walled hollow insulator, a metal sealing cap bonded across one end of said insulator, an elongated metal support member mounted on said cap and extending concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode surface on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, a coaxial substantially fiat-bottomed metal cuplike member having the rim thereof bonded to the outer surface of said insulator, an inward indentation disposed centrally in the bottom of said cup-like member, said indentation comprising an active electrode surface extending toward and cooperating with said active surface on the end of said support member to define a gap, and said active electrode surfaces being oppositely rounded and the gap therebetween being shorter than the distance between said active surface and said support member and the side wall of said cup-like member, thereby to localize arcing in said envelope between the central portions of said active electrode surfaces.

13. A gap device comprising a sealed envelope having as wall-defining elements thereof an elongated singlewalled hollow insulator, a metal sealing cap bonded across one end of said insulator, an elongated metal support member mounted on said cap and extending through said cap and concentrically through said insulator in spaced relation thereto throughout the full length of said support member, an active electrode surface supported on the end of said support member opposite said cap and disposed outwardly of the corresponding end of said insulator, a coaxial metal cup-like member having the rim thereof bonded to the outer surface of said insulator, the gap between said active electrode surface on said support member and the center of the bottom of said cup-like member being shorter than the distance between said electrode surface and the side walls of said cup-like member, said support member being substantially massive relative to the other metal parts of said device and said active surface on said support member being large relative to said corresponding end of said insulator, thereby to effect substantial thermal conducivity between the active electrode surface and said support member and to shield said corresponding end of said insulator from deposition of metal evaporated from said opposed metal surfaces.

14. A gap device comprising a sealed envelope containing a partially ionized quantity of ionizable medium and having as wall-defining elements thereof an elongated single-walled hollow ceramic insulator including a plurality of annular recesses in the outer surface thereof, a coaxial cup-like metal cap, a metallic bond sealing the rim of said cap to the external lateral surface of one end of said ceramic section, a support rod sealed through said cap and extending concentrically through said insulator in closely spaced relation thereto throughout the full length of said rod, an electrode element having a rounded active surface carried on the inner end of said rod and disposed outwardly of the corresponding end of said insulator, a conductive lead secured to the outer end of said rod, a coaxial substantially fiat-bottomed metal cup-like member, a metallic bond sealing the rim of said cup-like member to the outer surface of said ceramic section intermediate the ends thereof and with the inner end of said ceramic section extending a substantial distance into said cup-like member in spaced relation to the inner side walls thereof, a rounded inwardly extending intendation in the center of the bottom of said cup-like member cooperating with said electrode element to define a gap which is the closest spacing of opposed metal surfaces and the greatest region of field concentration in said device, the bottom of said cup-like member comprising a plate having a lateral extension for mounting purposes and heat dissipation, and a metal tabulation bonded to said plate for communicating with the interior of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,906,602 Hull May 2, 1933 2,194,145 Hansell Mar. 19, 1940 2,765,417 Foulke Oct. 2, 1956 2,804,573 Arrol et a1. Aug. 27, 1957 

